This invention deals with environmentally safe, non-toxic, protective coatings to provide corrosion resistance to metallic surfaces that are subjected to corrosive environmental conditions and/or thermal stress.
Painting of aluminum and steel structures in the aerospace/seacoast environment requires the use of separate primers and topcoat primers containing toxic and environmentally hazardous lead and chromium compounds. Such coatings often contain high levels of volatile organic compounds which may also be detrimental to the environment and human health. Application of two different coatings, primer and topcoat is labor intensive and also requires inventorying two types of materials thereby increasing the cost of using such coatings. Inspection and measurement of each coating further increases labor costs and is prone to error. Although existing single coat primer-topcoat paints provide some degree of remedy for some of these problems they also have extremely short pot life making them difficult to work with and do not have the level of corrosion inhibitive properties to satisfy launch vehicle requirements.
There is, therefore, a need for an environmentally safe, non-toxic, coating composition to protect aluminum and steel structures and machinery, such as those used in aerospace as well as other industries, from corrosion produced by exposure to environments such as the seacoast. Such a composition should be safe and efficient to use and require only one coat to provide effective corrosion protection. In addition, the coating composition should be substantially free of environmentally hazardous and toxic substances such as lead and chromium compounds and volatile organic compounds. A further requirement for such a coating is that it should have a long pot life to enable efficient use without an undue waste of material. Furthermore, should the coating become damaged, it should be easy to repair in an inexpensive, safe and convenient manner.
The present invention is directed to meeting the foregoing needs by providing a two part resin composition comprising a combination of resins containing non-toxic corrosion inhibiting pigments and low levels of volatile organic compounds. The coating composition has a pot life of about three to about four hours and excellent solvent resistance. The coating composition is applied as a single coat without the need of a primer coat and provides excellent corrosion protection to aluminum and steel substrates when exposed to salt spray. The first part or part A of the coating composition comprises saturated polyester polyols having an equivalent weight of about 100 to an equivalent weight of about 2500. The second part or part B of the coating composition comprises a polyisocyanate composition with the polyisocyanate composition selected from polyisocyanate compositions consisting of hexamethylene diisocyanate, the biuret of hexamethylene diisocyanate (HDI), the trimer of HDI and mixtures and reaction products thereof. Other compounds and polymers having reactive groups that can react with isocyanate functions such as epoxy resins can also comprise part A. The corrosion resistant composition comprising a polymeric matrix resulting, from the reaction of polyisocyanate composition and the polyol contains corrosion inhibiting pigments selected from the group consisting of zinc hydroxy phosphite, zinc phosphate, calcium ion-exchanged silica, calcium strontium zinc phosphosilicate and mixtures thereof and is applied to a substrate metal in one coat without the need of a primer.
The corrosion resisting coating of the present invention is formed from a two part resin composition comprising a combination of resins containing non-toxic corrosion inhibiting pigments and low levels of volatile organic compounds. The two parts of the coating composition, part A and part B, are mixed prior to application and react to form a strongly adherent solvent resistant polymeric matrix comprising a polyurethane that contains corrosion inhibiting pigments selected from the group consisting of zinc hydroxy phosphite, zinc phosphate, calcium ion-exchanged silica, calcium strontium zinc phosphosilicate and mixtures thereof and is applied to a substrate metal in one coat without the need of a primer. The pot life of the formulation after mixing is about three to about four hours. The first part or part A of the coating composition comprises a saturated polyester polyol comprising a reaction product of phthalic anhydride and trimethylol propane (also known in the art as TMP) having an equivalent weight of about 100 to an equivalent weight of about 2500. Preferably, the saturated polyester polyols comprising part A are characterized by an equivalent weight of about 225 to about 600, an acid number not greater than about 4 and a hydroxyl number of about 104 to about 220. Preferred saturated polyester polyols are selected from compositions that are characterized by an acid number (on solution) of about 0 to about 3, a hydroxyl number (on solution) of about 104 to about 120, with an equivalent weight (on solution) of about 468 to about 539 and an equivalent weight (on solids) of about 374 to about 431 with representative commercially available formulations marketed under the name Chempol(copyright) 18-2217 (Freeman Chemical Corporation, Port Washington, Wis.) and Mobay 670A-80 (Bayer, Mobay Corporation, Pittsburgh, Pa.) and compositions that are characterized by an acid number (on solution) of about 1 to about 4, a hydroxyl number (on solution) of about 155 to about, 190, with an equivalent weight (on solution) of about 312 to about 328 and an equivalent weight (on solids) of about 480 with representative commercially available formulations marketed under the name Chempol(copyright) 18-2230 (Freeman Chemical Corporation, Port Washington, Wis.) and Desmophen 650A-65 (Bayer, Mobay Corporation, Pittsburgh, Pa.) and compositions that are characterized by a maximum acid number of about 3, a hydroxyl number of about 200 to about 220, with an average equivalent weight of about 267 with a representative commercially available formulation marketed under the name Desmophen 631A-75 (Bayer Corporation, Industrial Chemicals Division, Pittsburgh, Pa.) and other equivalent compositions and formulations and mixtures thereof.
The second part or part B of the coating composition preferably comprises an aliphatic or cycloaliphatic polyisocyanate resin based on hexamethylene diisocyanate (also known in the art as HDI) having an average equivalent weight of about 255. A commercially available formulation comprising such a polyisocyanate based on HDI is Desmodur N-75 (Bayer, Mobay Corporation, Pittsburgh, Pa.), however other equivalent compositions and formulations may also be used. In practice the polyisocyanate comprises a biuret of HDI, but an HDI trimer can also be used. Other polyisocyanates such as those enumerated in U.S. Pat. No. 5,043,373, which is herein incorporated by reference, may also be used to formulate part B of the present composition.
Typical formulations for Part A and Part B of resin matrices for corrosion resistant coating compositions are given in Example 1, Table 1. Standard methods known in the art are used in preparing these resin matrices, for example those described in Hegedus et al, U.S. Pat. No. 4,885,324 which is herein incorporated by reference and the previously incorporated U.S. Pat. No. 5,043,373. Generally, the quantities of polyol and polyisocyanate are preferably adjusted to provide an approximately 1:1 ratio of hydroxyl to isocyanate groups when part A and part B are mixed. However, effective resin matrices can be obtained when the concentration of either of these functional groups in the mixture exceeds the other by about 20%. The following examples demonstrate the overall superior performance of the preferred polymeric matrix compositions which are derived from coating formulation 1 and coating formulation 3 in Example 1.